This invention relates to transmitting antennas and especially to such antennas adapted for electromagnetic interference testing of objects within a test chamber. Such electromagnetic interference or EMI testing is assuming a greater role in the design of electronic controls for motor vehicles as these controls increase in numbers and importance and as the number of possible EMI sources also increases. One type of EMI testing of motor vehicles is accomplished within a test chamber which isolates the testing environment from outside EMI sources and further requires an antenna within the test chamber to transmit the desired EMI signals within the test environment.
A problem with the test arrangement as described above is the limit imposed on the length of a resonant antenna element by the physical dimensions of the test chamber. As a practical matter, it is found that such a test chamber of reasonable size is too small to permit the use of resonant antenna elements which will transmit the lowest desired resonant frequency EMI signals. As an example, a room having a longest horizontal dimension of approximately 28 feet or 8.4 meters permits a resonant quarter-wavelength antenna element having a length of approximately 24 feet or 7.2 meters, and since the element should not approach the shielded walls too closely. This antenna element will transmit a signal having a wavelength no greater than approximately 29 meters or a frequency no lower than approximately 10 MHz. However, in EMI testing of motor vehicles, it is often desirable to transmit at frequencies down to 70 KHz.
Therefore, it is an object of this invention to provide a broadband transmitting antenna for a test chamber which is capable of transmitting signals in a full range of frequencies, with wavelengths extending beyond those possible for tuned elements within the physical dimensions of the test chamber.